Microelectronic packages called "multi chip modules" (MCMs) are constructed with unpackaged semiconductor dice. With a multi chip module, a number of dice are attached to a printed circuit board or other mounting substrate and electrically connected to form various circuits and electronic devices.
One reason for the increased use of multi chip modules is increased system performance. In particular, integrated circuits on multi chip modules can be operated with lower resistances and parasitic capacitances. This is largely a result of decreasing the interconnection length between the dice included in the multi chip module. In addition, overall system performance is improved because the input/output ports can be configured to access the whole module, which can be organized to reduce signal delays and access times. The power requirements are also reduced due to a reduction in the driver requirements.
Typically the unpackaged dice are mounted on a circuit board having an interconnect pattern formed using a process such as screen printing. Different techniques are used for mounting the dice to the board and for providing interconnection and termination of the unpackaged dice. One such technique is referred to as "flip chip bonding". With flip chip bonding, each die is mounted circuit side down, and bond pads on the die are bonded to corresponding connection points on the circuit board. Flip chips are formed similarly to conventional dice but require an additional process step to form solder bumps on the bond pads. Other techniques for mounting the unpackaged dice to the circuit board include wire bonding, tape automated bonding (TAB) and micro-bump bonding.
In general, with each of these methods the full surface area of the dice (i.e., circuit side or back side) occupies a corresponding area on the circuit board. As is apparent this uses a large surface area of the circuit board. It would be desirable to effect an interconnection between the dice and circuit board in which less surface area is required. This would allow a higher packaging density for the dice.
Another important consideration in constructing multi chip modules is the electrical connection between the bond pads of the unpackaged dice and the connection points on the circuit board. It is important that these electrical connections provide a low resistance or ohmic contact. It is also important to minimize the effect of thermal expansion on the electrical connections. For example, if the dice and the circuit board expand by a different amount, stress can develop at the connection points and compromise the connection.
In view of the foregoing, it is an object of the present invention to provide an improved method for mounting and electrically interconnecting unpackaged semiconductor dice to a circuit board.
It is a further object of the present invention to provide an improved method for interconnecting dice in which a high device density and a reliable electrical connection can be achieved.
It is yet another object of the present invention to provide an improved method for interconnecting dice which can be used with conventional semiconductor dice having flat bond pads.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.